This invention relates to a process for recovering alumina from high silica bauxite.
The Bayer process is the predominant conventional refining process for recovering alumina from alumina-bearing ore. The Bayer process involves digesting an alumina-bearing ore, such as bauxite, in caustic liquor under pressure to form an aluminarich solution. The solution then is separated from digestion residue, commonly called red mud, consisting of undissolved portions of bauxite as well as insolubles formed during digestion. The red mud typically is discarded as waste. Supersaturated alumina solution then is seeded with alumina and agitated to precipitate aluminum hydrate from the supersaturated liquor. Spent liquor is returned to the pressure digestion where its caustic values are used to treat more bauxite. Most of the high-purity alumina used in the electrolytic production of aluminum today is produced in this Bayer process by either low temperature digestion to extract gibbsite or under higher temperature to extract both gibbsite and boehmite.
Commercial operation of the Bayer process is confined to the treatment of bauxite containing less than about 6% reactive silica by weight. High silica bauxite, i.e., bauxite containing more than about 5% reactive silica by weight, cannot be treated economically by the Bayer process because of high soda losses. The higher reactive silica content raises the cost and decreases the efficiency of the process. Reactive silica in the bauxite affects the economics of the Bayer process. Reactive silica, e.g., present in the bauxite ore as kaolinite, causes soda losses in the Bayer process by the formation of a soda-silica-alumina desilication product. Soda costs attributable to desilication product are proportional to the reactive silica in the bauxite processed.
Various processes have been proposed for the commercial extraction of alumina from high silica, alumina-bearing ores. Brown in U.S. Pat. Nos. 2,375,342 and 2,375,343 proposed a sinter method for recovering alumina from low-grade ores by treating the low-grade ore to solubilize its alumina and to separate dissolved alumina from silica and other unwanted impurities in the ore. Alkaline earth and alkali metal compounds are mixed with ore and then sintered. The function of the alkaline earth compound is to insolubilize silica. Any compound which forms insoluble silicates could be used in the Brown process, but readily available and low-cost limestone is preferred. The amount of limestone to be added depends on the amount of silica in the ore. The sintered mixture is then leached to recover soluble alumina and caustic values. The leach liquor will also contain quantities of solubilized impurities, principally silica. The leaching media may be heated, preferably to temperatures not in excess of 200.degree. F., higher temperatures promoting the formation of insoluble complexes containing alumina. The residue of alkaline earth silicate and insoluble impurities (plus insoluble alumina) is discarded. Leach liquor containing substantial amounts of soluble silica is added to a bauxite-caustic liquor and digested in a conventional Bayer process.
Misra, U.S. Pat. No. 4,468,375, discloses that caustic soda reacts with silica minerals present in bauxite, and terms this a "chemical caustic loss" dependent upon the amount of caustic and reactive silica minerals present in the bauxite. Misra notes that the Bayer process alumina product generally contains various inorganic impurities such as silica, and the process efficiency is lowered by such impurities which accumulate in the caustic liquor as it recirculates through the initial step of bauxite digestion in the Bayer process. Misra proposed a process including comminuting aluminum-containing mineral ore; reacting the comminuted ore at an elevated temperature with an aqueous solution of at least 150 g/L of sodium bicarbonate to form a solid reaction product of dawsonite and impurities; and converting the dawsonite to alumina. In this way, an aqueous solution of sodium bicarbonate replaces aqueous sodium hydroxide in the initial bauxite digestion.
Oku et al., U.S. Pat. No. 3,716,617, discloses a process for producing alumina according to the Bayer process and separating digestion residue from sodium aluminate slurry. Although ambiguous, Oku refers to a "reactive silica" as silica present as clay and/or any other silicate in an alumina-containing ore. Oku discloses that when the temperature during extraction of an alumina component from bauxite is high, digestion time required may be short but, on the other hand, the rate of dissolution of the reactive silica becomes greater, so the rate of variation of the reactive silica content in the digestion residue becomes quick and the operation becomes difficult. Further, the desilication reaction is accelerated and the alumina and alkali solution are lost. On the other hand, if the digestion temperature is low, the desired high alumina concentration in the sodium aluminate solution cannot be obtained. Therefore, the digestion temperature is usually 90.degree. C. to 150.degree. C., preferably 110.degree. C. to 140.degree. C. The Oku process mixes bauxite containing over about 10% by weight total silica, 8.5% by weight reactive silica, with sodium aluminate solution which apparently is silica-free. The digestion residue is separated from the sodium aluminate solution by a synthetic organic high molecular weight flocculent, the separation being conducted only when at least 5% by weight reactive silica remains in the digestion residue. Soda concentrations higher than 70 g/L are disclosed for digestion.
Roberts, U.S. Pat. No. 3,413,087, discloses extracting alumina from bauxite by digestion in a caustic alkali solution in which the silica content is insolubilized. The Roberts process predigests all the bauxite in a caustic alkali solution insufficient to dissolve all the soluble alumina in the bauxite but sufficient to dissolve substantially all the silica in the bauxite at an elevated temperature and for a time sufficient to permit desilication of the liquor by precipitation of the silica in the form of insoluble complex sodium aluminum silicate. By maintaining the slurry in the predigestion stage at a temperature above about 150.degree. F., crystallization and precipitation of the silica form complex sodium aluminum silicate as desilication product. The crystals of desilication product act as a seed for promoting nucleation and precipitation of additional amounts of desilication product on previously formed crystal surfaces, in preference to deposition on heat exchange surfaces to improve heat exchange rates in the process.
The first output of alumina from Africa is described in Chemical Engineering, Oct. 2, 1961. Alumina is produced from low-silica bauxite at Fria, Guinea, by atmospheric digestion at about 220.degree. F. by digestion of the ore in a sodium aluminate liquor (200 g/L Na.sub.2 O).
Fish, U.S. Pat. No. 3,681,013, discloses a process for eliminating the erosion normally encountered during high pressure digestion of bauxite containing considerable amounts of non-reactive silica, e.g., such as quartz sand. The Fish process separates a coarse fraction from fines and digests the fines in a conventional pressurized digestion.
It is an object of the present invention to produce purified alumina from alumina-rich gibbsitic ore containing high amounts of reactive silica.
It is another object of the present invention to provide for the digestion of alumina from low-grade bauxite ores while inhibiting the dissolution of reactive silica from kaolinite in a sodium aluminate solution.
It is yet another object of the present invention to purify alumina-rich ore containing high amounts of reactive silica while minimizing the loss of soda in the form of desilication product.
These and other objects will become apparent from the disclosure which continues as follows.